Narrow-linewidth single-frequency optical fiber laser devices is an important branch of the development of laser devices, which have extremely narrow linewidth, low noise, and excellent coherence characteristics. Its optical spectrum linewidth is extremely narrow (which can be up to 10-8 nm), which is narrower than the linewidth of the currently best narrow linewidth DFB laser device by two orders of magnitude, and is narrower than the linewidth of the DWDM signal light source in current optical communication networks by 5-6 orders of magnitude. It is especially important because it shows substantive potential values to be applied to the technical fields of coherence optical spectrum beam, laser radiators and non-frequency transitions. In these technical fields, it is generally required that the optical spectrum linewidth of the laser device being extremely narrow and linear-polarization functioning and being tunable (operation in multi-channel or multi-wavelength). These parameters determine and affect the resolution, transition efficiency, costs of the applied situations. Therefore, there is a need for development of a simple and reliable tunable narrow-linewidth single-frequency linear-polarization laser device.
According to the recent research, it is reported that a tunable single-frequency laser device generally uses rare-earth-ion doped with quartz optical fiber or rare-earth-ion doped with solid crystal as the operating medium for the single-frequency laser. It inserts optical modulated crystal blocks with low reliability (such as electro-optical crystal, thermo-optic crystal, F—P etalons, etc.) in the optical path to maintain the single frequency operating or adjust the laser frequency. However, there are a series of problems, such as that the concentration of the doped rare-earth-ions cannot be further increased, the resonant cavity is too long, prone to mode hopping, has poor reliability, and a maximum output tunable single-frequency laser of tens mW magnitude. The biggest difficulty is that it is hard for the linewidth to be under 10 kHz, which leads to heavy noise.
Using rare-earth-ions heavily-doped multicomponent glass optical fiber as the gain medium of the laser, in conjunction with a short and straight single-frequency resonant cavity is able to effectively implement a single-frequency laser output with an output power more than 100 mW and a linewidth less than 2 kHz. Research related to this comprises a report on the erbium-ytterbium co-doped phosphate glass optical fiber with a length of 2 cm by C. Spiegelbert et al. which achieves a single-frequency optical fiber light output with an output power more than 200 mW and a linewidth less than 2 kHz and a wavelength of 1.5 μm [J. Lightwave Technol., 2004, 22: 57]; a report on the ytterbium-doped phosphate glass optical fiber with a length of 0.8 cm by Z. Feng et al. which achieves a single-frequency linear-polarization optical fiber laser output with an output power more than 20 mW, a linewidth less than 2 kHz, an extinction ratio more than 30 dB and a wavelength of 1.06 μm [Appl. Phys. Express, 2013, 6:052701]. In addition, a patent application of a high power narrow linewidth single-frequency laser system by The University of Alexandria and NP Photon etc. Inc. (Publication Number: U.S. Pat. No. 7,903,696 B2) utilizes two super short single-frequency resonant cavity output low power narrow linewidth single-frequency laser signals, the laser power of which being respectively amplified by an ordinary EDFA and a high power double-clad fiber amplifier. However, the optical fiber laser device required does not possess linear-polarization output with tunable wavelengths.